The invention relates to an arrangement according to the preamble of claim 1.
Arrangements of this type are known (Hewlett Packard Journal, December 1993, p 31 ff and p 47 ff, prior German Patent Application 197 57 296.0). With such an arrangement the transfer function of a measuring instrument, for example a spectrum analyser, from the input socket as far as the A/D converter, that is to say in respect of the entire analogue part of the measuring instrument, can be determined. As a result of evaluation by magnitude and phase of the measured values obtained in this way in the computer, the inverse function of the measured transfer function can be calculated and hence the transfer function of the measuring instrument can be compensated during the operation thereof. This process can be utilized both for narrowband and for very broadband analogue parts of measuring instruments. It is therefore also highly suitable for the calibration of intermediate-frequency stages (IF part) of measuring instruments that can no longer be adequately corrected in the baseband.
The calibrating and compensating process that can be performed with such an arrangement presupposes a signal at the A/D converter that, considered within the frequency range, represents the amplitude response and the group delay or phase within the bandwidth to be corrected. In general, this is obtained with a line spectrum that, for example, exhibits equidistant lines within the range of measurement. The spacing of the lines is chosen to be so small that the amplitude response and the phase response are modelled with sufficient precision by the lines.
In the arrangements of this type known hitherto the line spectrum is generated either by digital methods using a D/A converter (Hewlett Packard Journal, December 1993) or with the aid of a calibrating signal that increases in frequency over time and that is generated by multiplication of a monofrequency carrier signal by an auxiliary signal that increases in frequency in linear manner (Patent Application 197 57 296.0). These known devices for generating a line spectrum are relatively elaborate in terms of circuit technology; in addition, they have the disadvantage that, in the case of high bandwidth, the lines of the spectrum are generated in temporal succession and therefore a relatively long measuring-time is necessary.